In the long-term evolution infrastructure, a UE can be in one of two radio resource control (RRC) states. These are LTE_IDLE and LTE_ACTIVE.
The UE can be configured for discontinuous reception (DRX) in both the LTE_IDLE and the LTE_ACTIVE states. In the LTE_IDLE state, the DRX allows the UE to synchronize its listening period to a known paging cycle of the network. By synchronizing the listening period, the UE can turn off its radio transceiver during the standby period, thereby significantly saving battery resources. DRX parameters allow the mobile to synchronize with the network and to know that it will not receive another signal until a specified time has elapsed.
It is proposed at the 3GPP TSG-RAN working group that DRX also be used when the UE is in LTE_ACTIVE state. It is further proposed that when in discontinuous reception (DRX) by the user equipment (UE) in LTE_ACTIVE state, that a regular DRX configuration is signaled by a radio resource control (RRC) protocol message and a temporary DRX (re)configuration is signaled by medium access control (MAC) signaling, for example, in the MAC protocol data unit (MAC-PDU) header or MAC control PDU.
In LTE, before user data is sent on a downlink shared channel, the eNB will send a scheduling indication on the downlink shared control channel (DLSCCH) providing the parameters that the UE will use to demodulate the data. However, if the UE misses that DLSCCH scheduling indication, the UE will not know to receive the user data and, hence, will not know to acknowledge (ACK) or negatively ACK (NACK) the user MAC-PDU. The eNB will be awaiting the ACK or NACK, i.e. hybrid ARQ (HARQ), response. When it does not get that response, it is considered a discontinuous transmission (DTX). If the data indication on the downlink shared control channel (DLSCCH) is missed by the UE and the subsequent discontinuous transmit (DTX) on the HARQ feedback channel is misinterpreted as an ACK by the enhanced node B (eNB), a MAC-PDU may be lost. Moreover, if UE automatically increments DRX length according to a rule, e.g. no data activities in a certain duration, the eNB will maintain its current DRX value while the UE will increment its own DRX value. This results in DRX timing de-synchronization, i.e. the eNB and the UE are operating on different DRX periods.
As will be appreciated, this increases the downlink message delivery latency and wastes downlink radio resources. Longer delivery latency should be avoided, especially for critical downlink control messages. Once DRX timing de-synchronization occurs, the eNB has to determine the UE DRX timing in order to send new data to the UE. The eNB may achieve this by sending various probing messages to the UE at those times when it knows the UE may possibly be awake. It will, after a few attempts, find the UE DRX timing, and awaken the UE in order to regain synchronization.